Laser systems have been used in ophthalmic surgery for modifying the cornea of the patient. Systems such as shown in U.S. Pat. No. 4,729,372 to L'Esperance contemplate the controlled ablation of the cornea of the patient with a pulsed excimer laser. Operations performed with the system include corneal transplants and keratotomies.
The application of laser light to the cornea may be controlled by spot scanning of the cornea or by the use of masks. As shown in U.S. Pat. No. 5,108,388 to Trokel, the masks may, for example, employ slits or holes. Repeated scanning or pulsing through properly selected masks are employed to reshape or reprofile the curvature of the cornea to treat myopic or hyperopic conditions. The system can also be used, for example, to remove corneal sections for corneal replacements or transplants.
A system used by applicant for performing ophthalmic laser surgery is shown in FIG. 1. The system includes an Excimer laser 10 such as a COMPex 201 Excimer laser. An optical rail 12 contains optical elements for controlling the laser pulses and delivers spatially modulated pulses to a shuttling device 14, which acts as a selectively positionable turning mirror, for directing the laser pulses to a selected one of the two surgical stations, 16 and 18. The system allows surgery to be performed on one patient while a second patient is readied, and improves the utilization efficiency of the operating room, laser and optical rail.
FIGS. 2(a) and (b) are vertical and horizontal cross-sectional views and ray traces of an optical path which may be used in the system of FIG. 1 to deliver pulses from the laser 10' to the cornea of the patient at 20. A light beam from the laser is shaped and focused by a series of lenses 22, 24 and 26. A beam homogenizer 28 is located next in the optical path as shown. A spatial modulator 30 provides beam dimensions and orientations in accordance with predetermined treatment parameters appropriate for the surgery required by the patient. The spatial modulator may include a conventional iris and variable, slit mask(s) as well as controls for changing the axis of orientation of the mask(s). These systems are motor driven on command from a treatment computer containing a treatment algorithm into which the treatment parameters have been programmed.
The shuttling turning mirror 32 selectively directs the laser beam to one or the other surgical stations along one of the system arms 34 or 36 shown in FIG. 1. An imaging lens 38 is located in each arm. Pulses from the imaging lens are reflected by end turning mirror 40 toward the target area 42 on the patient's cornea.
It is important that pulses delivered to the cornea have the appropriate energy to ensure that the reprofiling, cutting or ablation produced is consistent with the prescribed treatment for the patient. Systems of the type shown in FIG. 2 have employed photo detectors selectively positionable in the main optical path of the system at the end turning mirror for the purpose of calibrating or adjusting the energy delivered by the system during a preliminary calibration phase. See U.S. Pat. No. 5,772,656 to Kloptek.
Other control systems have been proposed such as disclosed in U.S. Pat. No. 4,941,093 to Marshall et al., which includes a measurement device to measure the cornea surface profile and a feedback control system to control the laser operation in accordance with the measured and desired profiles. U.S. Pat. No. 5,423,801 to Marshall et al. discloses further control of the laser by a measurement signal from a beam-shaping means and/or cornea while it is exposed to irradiation by the laser. U.S. Pat. No. 4,973,330 to Azema et al. discloses a photo detector associated with a semi-transparent mirror, which is intended to furnish a treatment computer with information relative to the energy of the pulses exiting the laser before the laser beam reaches the controlling device. A laser calibration device is shown in U.S. Pat. No. 5,464,960 to Hall et al. which employs a phantom cornea with superimposed thin films of alternating colors.